Academic journal article Bulletin of the World Health Organization

Integrated Malaria Vector Control with Microbial Larvicides and Insecticide-Treated Nets in Western Kenya: A Controlled Trial/ Lutte Integree Contre Les Vecteurs Du Paludisme Au Moyen De Larvicides Microbiens et De Moustiquaires Impregnees D'insecticide Au Kenya Occidental : Essai Controle/ Lucha Antivectorial Integrada Contra la Malaria Mediante Larvicidas

Academic journal article Bulletin of the World Health Organization

Integrated Malaria Vector Control with Microbial Larvicides and Insecticide-Treated Nets in Western Kenya: A Controlled Trial/ Lutte Integree Contre Les Vecteurs Du Paludisme Au Moyen De Larvicides Microbiens et De Moustiquaires Impregnees D'insecticide Au Kenya Occidental : Essai Controle/ Lucha Antivectorial Integrada Contra la Malaria Mediante Larvicidas

Article excerpt

Introduction

A marked increase in malaria has recently been noted in the African highlands, largely due to the rise of drug-resistant strains of Plasmodium falciparum parasites. (1-4) The ecological features of the western highlands of Kenya support stable parasite transmission, and increasing population pressure has led to the clearance of natural swamps, massive deforestation and crop cultivation in the valley bottoms. (5,6) Because of these agricultural changes, many water bodies are now exposed to the sun and provide ideal conditions for vector proliferation and increased malaria transmission. (7) Malaria control in these highlands is based on insecticide-treated nets (ITNs), indoor-residual spraying (IRS) with insecticides and prompt and effective treatment of clinical malaria. (8) Many believe integrated vector management (IVM), targeting both larval and adult mosquitoes, is the future for malaria control. (9-11) While ITNs are currently the priority strategy, there is growing interest in attacking the aquatic stages of malaria vectors with microbial larvicides, in conjunction with environmental management. (5,6,12-21) Presently, microbial larvicides are being used in an operational context only in Dar es Salaam, in the United Republic of Tanzania. (22,23) However, in the future larvicide application will be expanded to many of the 15 countries included in the President's Malaria Initiative and this interest is likely to spread to other African countries. While IVM is often advocated,(10,11) there is little evidence from the recent scientific literature on the effectiveness of this approach.

We have demonstrated that microbial larvicides reduced malaria vector mosquito larvae and adult females by > 90% in a rural town in western Kenya. (19) However, the effectiveness of this approach for reducing the incidence of malaria among vulnerable children remains unproven. ITNs are a firm favourite in the armamentarium against malaria and their protective efficacy is well known, (24,25) with reductions of 40-70% in human mosquito biting rates. (23,26) In this study we used a "pre-test/post-test control group" design to investigate the contributions of both mosquito larval control with microbials and ITNs to reducing the burden of malaria in the Kakamega and Vihiga highlands of western Kenya, an area moderately endemic for the disease. (27)

Methods

Study area

Six highland valley communities (at an altitude of 1453-1632 m) were recruited in two densely populated districts in Kenya (28) (Table 1). These hilly areas were characterized by step-sided valleys with flat bottoms and plateaus, where most homes were built. Small streams ran along the valley bottoms and papyrus swamps were common. Valleys were approximately 2-4 [km.sup.2] in area and at least 1 km apart. Baseline entomological and clinical data were collected for 17 months (February 2004 to June 2005), after which microbial larvicide was applied to all aquatic habitats in three of the six study sites at weekly intervals for a further 19 months (July 2005 to January 2007). At the same time, larviciding was initiated by project staff in half the sites (Table 1) and ITNs were gradually introduced in all study communities by the National Malaria Control Programme through government health facilities and nongovernmental organizations (NGOs).

After baseline monitoring, three valleys were selected for Group 1, the larval control intervention (LCI), and another three for Group 2, the non-LCI group. Since the study valleys varied considerably in their entomological and epidemiological characteristics, the sites were grouped so that at baseline both groups were similar in the total number of aquatic habitats, the proportion of available habitats colonized by Anopheles larvae, the density and seasonality of adult mosquito household infestation, and malaria prevalence (Table 1). Group 2 was chosen as the control because one of the sites in the group bordered a malaria study site of the Kenya Medical Research Institute and we were afraid that treatment in our site would interfere with malaria transmission in the neighbouring site. …

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